MAGNETIC BEAMS Report Summary

Final Report Summary - MAGNETIC BEAMS (Magnetically manipulated molecular beams; a novel ultra-sensitive approach for studying the structure and dynamics of water surfaces.)

The “magnetic beams” project focused on developing and applying novel techniques for studying the dynamics and structure of surfaces. The limitations of conventional approaches make it particularly challenging and sometimes impossible to study the structure, chemical environment and motion of atoms and molecules interacting with a solid surface. One goal was developing a new instrument, the magnetic beam surface NMR spectrometer (MBSN). NMR which is so widely used in science, is not sensitive enough to be applied to a single layer of surface atoms/molecules, due to the low spin-polarization. The idea of the MBSN apparatus is to use magnetic fields to align the magnetic moments of gas phase molecules and then deposit them unto a cold surface where NMR experiments can be performed on the deposited surface layer. The approximately 100% spin-polarization of the molecules which can be achieved with this method, has the potential to increase the NMR sensitivity by 5 orders of magnitude and make single surface layer NMR experiments possible. During the project we developed a MBSN setup characterized by a relatively large flux of high spin-purity molecules combined with a sensitive NMR spectrometer. The performance of our setup is sufficient to detect a NMR signal from a single layer of hyper-polarized molecules, if the molecules maintain their spin-polarization for long enough (approx. 10 minutes). Initial MBSN measurement attempts on a layer of amorphous solid water (ASW) revealed surprisingly fast spin relaxation of the molecules after adsorption. We performed the first NMR spin-relaxation study of bulk ASW samples obtaining insight into the origin of this fast relaxation process. Future studies will exploit the MBSN setup we constructed to search for surfaces / conditions where spin relaxation rates are slower and NMR signal from single layers can be detected. A second instrument development goal was designing, building and using a second-generation helium spin echo (HeSE) spectrometer. This instrument is a helium diffractometer which uses magnetic fields to separate the quantum mechanical wave functions of different nuclear spin states in time and in space, enabling measurements of atomic-scale motion on surfaces on a pico–nano second time scale, a time-scale that is beyond the reach of conventional surface science tools. After succesfully constructing the instrument, we used it to study water structures on a gold and copper surfaces, study the interaction of helium atoms with a NaCl surface, measure the non-isotropic collective motion of atoms on a stepped surface and other surface systems. Finally, an additional unplanned outcome of the project was the development of a novel method which uses the magnetic fields of the HeSE setup to control and measure molecular rotational orientation states. Using this method we coherently controlled the rotational orientation of a hydrogen molecules before and after they collided with a copper surface. This results paves the way for a wide range of new quantum state-to-state scattering measurements on ground state molecules which can now be performed.